The present invention relates to a photoacoustic signal detecting device for detecting information relative to the surface and inside of a sample using photoacoustic effect.
The photoacoustic effect, which was discovered by Tyndall, Bell, Rontgen, et al. in 1881, represents the following phenomenon. When intensity-modulated light (intermittent light) is irradiated to a sample, heat is generated in a light absorption region and periodically diffused through a heat diffusion region so that the thermal distortion wave thus generated provides an ultrasonic wave. By detecting this ultrasonic wave i.e. a photoacoustic wave by a microphone (acoustic-electric converter) or a piezoelectric element to obtain the component in synchronism with the incident light, information relative to the surface and inside of the sample can be obtained. Particularly, by varying the modulation frequency, information in the depth direction of the sample can be obtained. A technique for detecting the above photoacoustic signal is disclosed in "HIHAKAI KENSA", Vol. 36, No. 10, pp. 730-736, October 1987 (Showa 62). However, this technique has the following problem in view of detecting the internal information of a sample in a non-contact non-destruction manner. In the system of using a microphone, in order to enhance the measurement sensitivity, the measurement must be carried out under the state that a sample is placed in a sealed chamber with the size of generally several cm's or so. Thus, the sample size will be limited to 1 to 2 cm. As the case may be, the sample must be cut. In the case of using a piezoelectric element, the piezoelectric element must be bonded to the back surface of a sample. Thus, the detecting sensitivity will be greatly varied according to the contact degree.
Then, a system of using light interference has lately attracted considerable attention as a technique of detecting a photoacoustic signal. This system is discussed in IEEE; 1986 ULTRASONIC SYMPOSIUM--pp. 515-526 (1986). Now referring to FIG. 7, a system using light interference will be explained in the case where a laser is used as a light source.
A parallel light emitted from a laser 1 is intensity-modulated an acoustic-optical modulation element (AO converter) 2. The thus obtained intermittent light is expanded to a beam of a desired diameter by a beam expander, which is thereafter focused on the surface of a sample 6 placed on an XY stage 5 by a lens 4. Then, the heat distortion wave created at a focusing position 130 generates an ultrasonic wave and also provides a minute displacement in the sample surface. This minute displacement will be detected by a Michelson interferometer explained below. A parallel light emitted from a laser 7 is expanded to a beam of a desired diameter by a beam expander 8. This beam is separated into two optical paths by a beam splitter 9. The one is focused on the focusing position 130 on the sample 6 by a lens 10 whereas the other is irradiated to a reference mirror 11. Then, the light reflected from the sample 6 and the light reflected from the reference mirror 11 interfere with each other. The interference pattern thus formed is focused on a photo-electric converting element 13 (e.g. photodiode) through a lens 12 to provide a photo-electric-converted interference intensity signal. This interference intensity signal is amplified by a preamplifier 14 and thereafter sent to a lock-in amplifier 16. The lock-in amplifier 16, using as a reference signal a modulated frequency signal from an oscillator 15 used for driving the acoustic-optical modulation element 2, extracts only the modulated frequency component contained in the interference intensity signal. This frequency component has information relative to the surface or inside of the sample according to the frequency. Therefore, if there is a defect such as a crack inside the sample, the modulated frequency component in the interference intensity signal provides a signal change so that the presence of the defect can be noticed. An XY stage shifting signal and an output signal from the lock-in amplifier 16 are processed by a computer 17. Accordingly, the photoacoustic signals corresponding to the respective positions on the sample are displayed on a display (e.g. a monitor television) 18 as image information.
The above mentioned prior art using light interference is a very efficient technique in that it enables a photoacoustic signal to be detected in a non-contact and non-destruction manner, but also has the following problem.
The transverse resolution of the photoacoustic signal is decided by the spot diameter on the sample of the laser beam from the laser 1 and the heat diffusion length of the sample. The heat diffusion length is a value peculiar to the sample whereas the spot diameter d is expressed by EQU d=1.22 .lambda.f/D (1)
where .lambda. is the wavelength of the laser beam, D is the beam diameter after having passed the beam expander, and f is the focal length of the lens 4.
However, generally, the light intensity distribution of the beam spot on the sample 6 has a sectional shape as shown in FIG. 8. Namely, there are high order diffraction components 20a and 20b around a peak portion having the diameter of d. Generally, the light intensity of the high order diffraction components is as high as 16.2% in that of the entire spot. If a minute displacement on the sample surface is detected through light interference, this value can not disregarded so that the transverse resolution of the photoacoustic signal is finally reduced. This applies to the spot diameter of the laser beam from the laser 7 for an interferometer. Namely, the transverse resolution of the interference intensity signal is reduced. Particularly, the reflection light of the high order diffraction component, when being incident to the photo-electric converting element 13, provides noise, which greatly reduces the detection sensitivity of the photoacoustic signal.